1: /* 
  2: 
  3: The following is an interesting snippet of code I wrote recently. It 
  4: makes a data pipe between a listen port on the machine it's being run on 
  5: and a port on a remote machine. For example, running
  6:   datapipe 2222 23 your.machine.com
  7: 
  8: would create a port 2222 on the local machine that, if telnetted to, would
  9: be the same as telnetting to port 23 on your.machine.com. This can be used
 10: for a variety of purposes: redirect IRC connections so that identd shows
 11: the username of the datapipe process; redirect sendmail direct connections
 12: for the same reason; even use on a firewall machine to give access to an
 13: internal service (ftpd, for instance). Cascaded datapipes make for
 14: interesting traceback dilemmas. Questions and comments accepted.
 15: 
 16: Compile with:
 17:     cc -o datapipe -O datapipe.c
 18: On boxes without strerror() (like SunOS 4.x), compile with:
 19:     cc -o datapipe -O -DSTRERROR datapipe.c
 20: 
 21: Run as:
 22:     datapipe localport remoteport remotehost
 23: 
 24: It will fork itself into the background.
 25: 
 26: /*
 27:  * Datapipe - Create a listen socket to pipe connections to another
 28:  * machine/port. 'localport' accepts connections on the machine running    
 29:  * datapipe, which will connect to 'remoteport' on 'remotehost'. Fairly
 30:  * standard 500 xxxx extended errors are used if something drastic
 31:  * happens.
 32:  *
 33:  * (c) 1995 Todd Vierling
 34:  *
 35:  * Define STRERROR while compiling on a SunOS 4.x box
 36:  */
 37: 
 38: #include <sys/types.h>
 39: #include <sys/socket.h>
 40: #include <sys/wait.h>
 41: #include <netinet/in.h>
 42: #include <stdio.h>
 43: #include <stdlib.h>
 44: #include <errno.h>
 45: #include <unistd.h>
 46: #include <netdb.h>
 47: 
 48: #include <linux/time.h>
 49: 
 50: #ifdef STRERROR
 51: extern char *sys_errlist[];
 52: extern int sys_nerr;
 53: char *undef = "Undefined error";
 54: 
 55: char *strerror(error)  
 56:   int error;  
 57: { 
 58:   if (error > sys_nerr)
 59:     return undef;
 60:   return sys_errlist[error];
 61: }
 62: #endif
 63: 
 64: main(argc, argv)  
 65:   int argc;  
 66:   char **argv;  
 67: { 
 68:   int lsock, csock, osock;
 69:   FILE *cfile;
 70:   char buf[4096];
 71:   struct sockaddr_in laddr, caddr, oaddr;
 72:   int caddrlen = sizeof(caddr);
 73:   fd_set fdsr, fdse;
 74:   struct hostent *h;
 75:   struct servent *s;
 76:   int nbyt;
 77:   unsigned long a;
 78:   unsigned short oport;
 79: 
 80:   if (argc != 4) {
 81:     fprintf(stderr,"Usage: %s localport remoteport remotehost\n",argv[0]);
 82:     return 30;
 83:   }
 84:   a = inet_addr(argv[3]);
 85:   if (!(h = gethostbyname(argv[3])) &&
 86:       !(h = gethostbyaddr(&a, 4, AF_INET))) {
 87:     perror(argv[3]);
 88:     return 25;
 89:   }
 90:   oport = atol(argv[2]);
 91:   laddr.sin_port = htons((unsigned short)(atol(argv[1])));
 92:   if ((lsock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) == -1) {
 93:     perror("socket");
 94:     return 20;
 95:   }
 96:   laddr.sin_family = htons(AF_INET);
 97:   laddr.sin_addr.s_addr = htonl(0);
 98:   if (bind(lsock, &laddr, sizeof(laddr))) {
 99:     perror("bind");
100:     return 20;
101:   }
102:   if (listen(lsock, 1)) {
103:     perror("listen");
104:     return 20;
105:   }
106:   if ((nbyt = fork()) == -1) {
107:     perror("fork");
108:     return 20;
109:   }
110:   if (nbyt > 0)
111:     return 0;
112:   setsid();
113:   while ((csock = accept(lsock, &caddr, &caddrlen)) != -1) {
114:     cfile = fdopen(csock,"r+");
115:     if ((nbyt = fork()) == -1) {
116:       fprintf(cfile, "500 fork: %s\n", strerror(errno));
117:       shutdown(csock,2);
118:       fclose(cfile);
119:       continue;
120:     }
121:     if (nbyt == 0)
122:       goto gotsock;
123:     fclose(cfile);
124:     while (waitpid(-1, NULL, WNOHANG) > 0);
125:   }
126:   return 20;
127: 
128:  gotsock:
129:   if ((osock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) == -1) {
130:     fprintf(cfile, "500 socket: %s\n", strerror(errno));
131:     goto quit1;
132:   }
133:   oaddr.sin_family = h->h_addrtype;
134:   oaddr.sin_port = htons(oport);
135:   memcpy(&oaddr.sin_addr, h->h_addr, h->h_length);
136:   if (connect(osock, &oaddr, sizeof(oaddr))) {
137:     fprintf(cfile, "500 connect: %s\n", strerror(errno));
138:     goto quit1;
139:   }
140:   while (1) {
141:     FD_ZERO(&fdsr);
142:     FD_ZERO(&fdse);
143:     FD_SET(csock,&fdsr);
144:     FD_SET(csock,&fdse);
145:     FD_SET(osock,&fdsr);
146:     FD_SET(osock,&fdse);
147:     if (select(20, &fdsr, NULL, &fdse, NULL) == -1) {
148:       fprintf(cfile, "500 select: %s\n", strerror(errno));
149:       goto quit2;
150:     }
151:     if (FD_ISSET(csock,&fdsr) || FD_ISSET(csock,&fdse)) {
152:       if ((nbyt = read(csock,buf,4096)) <= 0)
153: 	goto quit2;
154:       if ((write(osock,buf,nbyt)) <= 0)
155: 	goto quit2;
156:     } else if (FD_ISSET(osock,&fdsr) || FD_ISSET(osock,&fdse)) {
157:       if ((nbyt = read(osock,buf,4096)) <= 0)
158: 	goto quit2;
159:       if ((write(csock,buf,nbyt)) <= 0)
160: 	goto quit2;
161:     }
162:   }
163: 
164:  quit2:
165:   shutdown(osock,2);
166:   close(osock);
167:  quit1:
168:   fflush(cfile);
169:   shutdown(csock,2);
170:  quit0:
171:   fclose(cfile);
172:   return 0;
173: }